Various automotive components such as body panels, moldings, bumpers, and like are increasingly being fabricated from flexible plastics such as by reaction injection molding, compression molding of sheet molding compounds and injection molding of thermoplastic elastomers, etc. These flexible plastic automotive parts are generally provided with a smooth high gloss finish or top coat which is adhered to the flexible plastic part by an intervening primer or base coat. Such primer compositions should exhibit a number of desirable characteristics. In particular the primer composition should have good sprayability and flow properties, and be capable of quickly drying to form a flexible, opaque film having properties such as smoothness and chemical resistance which provide a base onto which a subsequent top coat having good reflective clarity and distinctness of image can be formed. The film must also exhibit excellant adhesion to the flexible plastic substrate and to a subsequently applied top coat. In order to reduce environmental health risks, the primer composition should preferably have a low volatile organic content. Additionally the primer composition should be capable of drying to form a base coat which exhibits good sandability properties, particularly when used during the repair of flexible plastic parts which have been damaged such as in a collision.
Various aqueous based or water-dispersable polymeric coating compositions containing acrylic and urethane type polymers are known to the art and described in the literature. However, none of the known compositions are specifically adapted for use as a primer coat for flexible plastic substrates. Specifically, the prior art compositions do not simultaneously exhibit all of the qualities which are desirable for a primer coating composition for flexible plastic substrates.
U.S. Pat. No. 3,360,494 to Bolinger, for example, discloses a textile size composition containing a water-soluble acrylic type polymer and a water-soluble polyurethane carried in an aqueous medium. This composition is unsuitable for use as a coating composition on account of its low solids content. Moreover Bolinger discloses a heat cured composition which is not suitable for air drying at ambient conditions.
U.S. Pat. No. 3,862,074 to Hickey relates to an aqueous dispersion of one or more acrylic polymers and one or more urethane polymers. This composition is disclosed as being suitable for use as a primer for metal and other materials. The disclosed compositions however do not simultaneous achieve all of the desired characteristics such as flexibility, opacity, good top coat hold out, and other desirable film forming characteristics, good sandability, and other properties which are desirable for a primer coating composition for flexible plastic substrates.
Various other references such as U.S. Pat. Nos. 4,103,050 to Laskin et al.; 4,298,511 to Schimmel et al; 4,339,365 to Becher et al; 4,430,367 to Lat; 4,507,426 to Blake Jr.; 4,522,958 and 4,526,910 to Das et al; 4,880,867 to Gobel et al; 4,927,876 to Coogan et al; 4,968,536 to Goldner et al; and 5,011,881 to Fujii et al disclose aqueous or water-based coating compositions containing an acrylic type polymer and a urethane type polymer. None of the foregoing references however teach all of the essential features such as total solids content, total resin content, ratio of urethane polymer to acrylic polymer, appropriate pigments and organic co-solvents, and other characteristics needed to simultaneously achieve good sprayability and flow properties, rapid air drying at ambient conditions, flexibility, opacity, appropriate coalescent and film forming properties, good adhesion to both the substrate and top coat, good sandability, the ability to fill scratches and minor imperfections in the substrate, and other properties which are desirable for a primer composition intended for application to a flexible plastic substrate, while simultaneously achieving a relatively low volatile organic content.